One of the major problems in foundry practice is to produce castings whose shape is as close as possible to that of the finished part. This is especially true of the production of large-size thin-walled castings, a process in which a maximum accuracy means a maximum efficiency, a reduced mass of castings, and a high rigidity and excellent vibration resistance of individual parts and whole structures.
However, the production of large-size thin-walled castings involves a number of difficulties of which the most serious one is the operation of filling a narrow cavity with molten metal. In an attempt to solve this problem, specialists have evolved a method known as casting by squeezing. Unlike all conventional casting techniques which make use of stationary molds, casting by squeezing is done in molds with cavities of a gradually diminishing cross-sectional size. This feature solves the basic problem of maintaining the original high temperature and fluidity of the compact mass of melt throughout the relatively short period of time during which the cavity of the mold is filled with melt.
In the case of stationary molds, the melt encounters a rapidly growing hydraulic resistance as it leaves the gating system and enters the narrow cavity of the mold. The situation is further aggravated by a decreasing temperature and increasing viscosity of the melt. The combined action of these factors may put the flow of molten metal to a stop.
Sand molds do not make it possible to produce castings with a wall thickness of 4 mm and a length of 200 to 300 mm. Metal molds do not make it possible to produce castings with a wall thickness of 3.5 mm and a length of 400 to 500 mm. Die casting does not make it possible to produce castings with a wall thickness of 1.5 to 2 mm and a length of 400 mm.
Unlike the above methods, casting by squeezing makes it possible to produce castings with a wall thickness of 2 to 2.5 mm and a length of 800 to 1,200 mm, or walls 2.5 mm thick and 800 to 1,000 mm in diameter.
In some cases casting by squeezing makes it possible to produce castings with a wall thickness of 2.5 to 3.5 mm and a length of 900 to 2,000 mm, an achievement which shows that casting by squeezing is a unique and highly promising process.
There is known a squeezing caster (cf. Sheet 28, FIG. 3 of "Liteyniye formy dlya tsvetnykh splavov"/"Casting Molds for Nonferrous Metal Alloys"/, Machinostroyeniye Publishers, Moscow, 1981) comprising a bed with a bottom plate which carries a rod for forming the internal surface of a casting, and two mold halves movable along the bed. The mold halves are driven by two power cylinders. The rod of each power cylinder is secured to the bed, while its barrel is mounted on a crosspiece. Each of the two mold halves is mounted on a respective crosspiece. The mold is preheated by heating elements arranged on the bottom plate and in the mold halves. There are two more power cylinders intended to separate a finished casting from the mold halves. Two cheeks are hingedly mounted on the sides of one mold half, each being movable in the angular direction. The cheeks are driven by a double-arm lever which has its first arm connected to the cheek and its second arm connected to the cylinder rod which, in turn, is hingedly connected to the crosspiece.
The caster in question is disadvantageous in a poor balance and frequent misalignments of the movable dies and in an excessive and nonuniform wear of the dies, bed and cheeks. The poor balance and frequent misalignments of the dies are due to the fact that the force transmitted from the hydraulic cylinder to the die is applied at one point. As the mold halves are brought together and their internal cavities are filled with melt, reacting forces are produced. The distribution of these forces is not uniform over the height of the mold, and their curve changes as the mold halves are brought together and the melt level rises in the mold cavities. The more remote the resultant of the reacting forces from the point at which the force produced by the hydraulic cylinder is applied, the greater the bending moment which destabilizes the dies. The distribution of the reacting forces is also nonuniform over the width of the mold; the resultant bending moment causes a misalignment. The above factors affect the dimensional accuracy of castings and account for rapid wear and frequent seizures of the dies.
There is known a squeezing caster according to USSR Inventor's Certificate No. 634,848, comprising a bed which carries a movable split-type intermediate carrier member and a bottom plate. The latter carries a rod intended to form the internal surface of a casting. The caster also incorporates two dies which are movable along the intermediate carrier member. The dies are driven by drives, each accommodated in a housing. The drive housing is mounted on the intermediate carrier member.
The dies are mounted on crosspieces which interact with the die drives.
Each of the cheeks is rigidly mounted on the intermediate carrier member and the housing. The caster is also provided with a drive for the intermediate carrier member. One of the drives brings the parts of the intermediate carrier member into abutting relation, after which the other drive brings the dies to their final position. As a result, the distance covered by the movable parts of the squeezing caster in contact with molten metal is reduced to a minimum of 10 to 12 mm, which considerably reduces wear of the bed and dies. However, that does not eliminate deformations and poor stability of the dies.
As the dies are brought closer together and the melt fills their cavities, reacting forces are produced. The distribution of these forces is not uniform over the height of the mold, and their curve changes as the dies are brought together and as the level of molten metal rises in their cavity. The more remote the resultant of the reacting forces from the point at which the force produced by the hydraulic cylinder is applied, the greater the bending moment which destabilizes the dies. The distribution of the reacting forces is also nonuniform over the width of the mold; the resultant bending moment causes a misalignment. The above factors affect the dimensional accuracy of castings and lead to rapid wear and frequent seizures of the dies.